The invention relates to a DC power source with a smoothing circuit comprised of a DC reactor and a parallel capacitor and, more particularly, to a DC power source protected from an excessive current produced in load short-circuiting.
In a conventional DC power source shown in FIG. 1, an AC output from an AC power source 1 is converted into a DC output by a thryristor rectifier 2 comprising thyristors S1 to S6. Then, the pulsating components in the DC output are filtered out by a smoothing circuit comprising a DC reactor 3 and a parallel capacitor 4 and the smoothed DC output is outputted from terminals 5 and 6. The DC power source unit shown in FIG. 1 is often used in such a manner that the terminals 5 and 6 of the power source unit is coupled with terminals of an inverter 7 comprising main thyristors S7 to S10 and flywheel diodes D1 to D4 thereby feed AC power to a load 10.
In such a case, as a load power factor of the inverter 7 is smaller, the output current of the DC power source apparatus includes more ripple component. It is for this reason that the ripple voltage appearing between the terminals 5 and 6 is reduced by increasing the capacitance of the capacitor 4. For example, an inverter of 100 KVA is coupled with a DC power source of DC 200 V. In this case, about 100,000 F is necessary for the capacitor 4. In the DC power source with such a large capacitor 4, when a commutation failure takes place in the inverter 7, the terminals 5 and 6 shortcircuit the thyristors S7 and S9 or S8 and S10 in the inverter 7 so that an excessive discharging current flows into the shortcircuited thryristors, thus possibly resulting in destruction of the thyristors.
One of the proposals to solve this problem is disclosed in Japanese Publication No. 45134/'72, in which, in the commutation failure in the inverter, the thyristors forming the inverter are all made conductive and thus an excessive current discharging from the capacitor is distributed into a plurality of current paths so that current flowing into a single thyristor is reduced. Another proposal uses a DC fuse between the inverter and the discharging loop thereby to restrict current flowing therethrough.
In the former proposal, there are needs for a reactor for restricting an excessive current from the capacitor into a discharging loop and for a surge absorber for absorbing a spike voltage occurring at the commutation in the reactor. The capacitor provided at the DC output terminal side reduces an impedance at the DC power source side. The provision of the reactor is counter to the end of the use of capacitor. Therefore, it is limited in the increasing value of reactor inductance, so that the restriction of the discharging current is insufficient, for example, in the inductor of approximately 100 KVA, discharging current is large, 30 KA. In the latter proposal, there has not yet been developed a high speed switch adaptable for a very high DC voltage, for example 1.5 to 20 KV. In an ordinary high speed fuse, at least three msec is necessary to complete its shutting off operation, and therefore there is a possibility that it fails to protect the load. In some load, when load shortcircuiting frequently takes place, the power source apparatus must start again immediately after 100 .mu.sec. However, the present fuse can not do such an operation. Anyway, conventional proposals have not fully been successful in overcoming problems caused by an excessive current at an accident.